A Danish 275 and a German 644 N. Please note, that the 275 has a flaw; the goldring completing the passage from the blind cap to the body of the pen is missing.
If you look closely at the inscription on the bottom af the caps, you will notice two completely different font styles. The 664 is the pen in the foreground. Also note that the 275 has vertical lines on the clip whereas the 644 is smooth.
The nibs. The 275 to the left. I don't know if the nib in the middle is original to the 644N or whether it's a substitution from a 254. It seems more rounded and less pointed than the 200 series Flügelfeder.
The barrel of the 644N has a transparent, red striped part. The 275, on the contrary, has a solid barrel.
As you can tell the 275 is a button filler, and the 644N is a piston filler.
Differences in feed design. The 275 uses a more classic feed, and the 644N shares feed design with the 200 series.